Communication satellites furnish signals to large geographical areas on the ground. Specially adapted antenna designs make it possible to provide signal coverage for a particular reception area or country. An antenna system comprising one or a plurality of feed horns that transmit microwave radiation toward a reflector with a shaped reflecting surface can be used to achieve satisfactory receiver performance.
By creating satellite antenna systems with dual reflectors it is possible, at relatively low additional cost, to coordinate two separate antennas into a single antenna system. The respective reflectors are equipped with individually designed reflecting surfaces. The reflectors are arranged as upper and lower reflectors at a specific distance from one another. Fastening to the satellite is achieved by using the same fastening mechanism for the two reflectors, which are otherwise substantially independent of one another. Each reflector comprises a reflecting surface that reflects incident radiation from the feed horns with a specific direction of polarization.
In order for incident radiation to be able to penetrate down to the underlying reflector, these antenna systems are designed as “dual-grid” antennas. Such antennas are characterized in that the upper reflector consists of a partially RF-transparent structure that reflects only radiation that exhibits a particular direction of polarization. Two feed horns aimed toward the reflectors emit microwave radiation with orthogonal directions of polarization. These reflectors are arranged in such a way that the microwaves from a first feed horn are reflected by an upper reflector, which is arranged so as to reflect microwaves having the direction of polarization derived from the first feed horn. A second feed horn emits microwaves with a direction of polarization that is orthogonal to that of the radiation from the first feed horn. As a result, substantially all of the radiation from the second feed horn passes the first reflector and is reflected by a second, underlying reflector. The polarization sensitivity of a first reflector that is aimed toward the feed horn is achieved in that a band lattice structure is applied to the surface of the reflecting dielectric reflector. This lattice structure can consist of, e.g. a plurality of parallel copper conductors coating a substrate. The distance between the lattice band on one reflector surface is made small enough to cause the incident microwave energy, whose electrical field is polarized parallel to the lattice band of the reflecting surface, to be reflected. The second reflector can also include a reflecting surface with a polarization-sensitive lattice structure, or a surface that reflects all the incident radiation. Conventional dual-grid antennas use reflectors made of a dielectric material such as Kevlar.
Antennas with polarization-sensitive reflection are known from numerous publications. German patent DE, C 958 668 describes a reflector antenna with dual reflecting surfaces. The reflecting surfaces described in the patent consist of a number of conductors comprised of metal rods oriented in parallel with the reflecting direction of polarization. However, this type of antenna cannot be used under the special conditions imposed by in-space applications, since far too many rods would be needed to achieve sufficient reflection in the relevant frequency band. This antenna is also unable to meet in-space requirements in terms of rigidity and thermal stability.
U.S. Pat. No. 2,790,169 describes yet another antenna for polarization-sensitive reflection from two reflector elements. The first reflector element consists of a flat or parabolic bottom plate. The second reflector element consists of a number of parallel conductors that are fixedly mounted in the bottom plate at a uniform distance from one another. Complete reflection of radiation with a particular degree of polarization can be achieved by adjusting the extent of these conductors above the bottom plate (the depth of the conductors), and their mutual separation. This antenna assumes contact among the conductors that together comprise the first reflector element and the underlying reflector. One problem with this antenna is that the design of the reflector elements results in high transmission losses. Nor can this antenna be adapted to meet in-space requirements in terms of rigidity and thermal stability.
U.S. Pat. No. 4,625,214 describes a conventional structure for satellite antennas with dual reflecting surfaces. The parabolic reflectors are constructed on the basis of honeycomb cores of, e.g. Kevlar material. The lattice pattern consists of parallel metal ribs that are fixedly secured to the surface of the parabolic reflector. This known design functions satisfactorily up to the KU band (12 GHz−22 GHz). The density required for the metal pattern at higher frequencies makes this technology difficult to use at frequencies above the KU band. The design and choice of materials also result in reduced electrical performance and dielectric losses.